Method for drying wood shavings



May 12, 1970 H. KLINKMUELLER ETAL 3,

METHOD FOR DRYING W001) SHAVINGS Filed Nov. 15, 1967 2 Sheets-Sheet 1 FIG ATTORNEY May 12,1970 H. KLINKMUELLER ETAL 3,510,956

METHOD FOR DRYING WOOD SHAVINGS 2, Sheets-Sheet 2 Filed Nov. 15, 1967 N UR United States Patent M METHOD FOR DRYING WOOD SHAVINGS Horst Klinkmueller, Stuttgart, and Friedrich Hirth, Stuttgart-Feuerbach, Germany, assignors to Gann Apparateund Maschinenhau G.m.b.H., Stuttgart, Germany Filed Nov. 15, 1967, Ser. No. 683,295 Int. Cl. F26b 3/02 US. Cl. 34-25 7 Claims ABSTRACT OF THE DISCLOSURE A process for drying wood shavings in the manufacture of pressed boards or lumber. The moisture content of the wood shavings is measured at the beginning and end of the process. The temperature of the process is adjusted as a function of the moisture content of the wood shavings at the beginning of the process. The speed of conveyance at which the wood shavings are conveyed through the process is regulated as a function of the moisture content of the shavings at the end of the process. A mass of air is passed through the wood shavings and brought into contact with them to the extent that the moisture in the air achieves an equilibrium state wih the moisture in the shavings. A fresh supply of air is added to this mass of air so as to regulate the humidity of this air mass as a function of the moisture content in the air when in equilibrium with the moisture content of the wood shavings. The regulating functions are accomplished through motorized feedback loops. The moisture measuring instrument includes an auger which forces a sample of the wood shavings into a measuring zone where the electrical resistance of a compressed sample of the wood shavings is measured by means of electrodes projecting into the instrument chamber. The electrical resistance is an indication of the moisture content of the wood shavings.

BACKGROUND OF THE INVENTION The manufacture of boards made from compressed sawdust, has become increasingly important throughout the 'world. The production process beginning with the production of the sawdust or wood pulp to the point where the raw material is compressed into finished boards, is highly automated. In this continuous manufacturing process, however, attempts, heretofore, have been unsuccessful for automating as well the drying process.

In the production of boards from sawdust or wood pulp, it is essential that the proper moisture constant be maintained in order that the product have the desired quality and that the process during manufacture is carried out in a smooth and unobstructed manner. The regulation of the moisture content of the wood pulp at the output of the dryer has, heretofore, presented considerable difiiculties. In particular, it has not been possible, in the past, to maintain the moisture content down to 2% and to measure this amount of moisture with the required accuracy. For this reason, all attempts to automate completely the production process for the compressed wood pulp boards, have failed.

For purposes of measuring the moisture content of the W006. pulp, a variety of processes and arrangements have been proposed. Three processes or apparatus are of principal importance. These may be enumerated as follows:

(a) resistance-based measurements (b) measurements based on the dielectric constant (c) measuring the relative humidity of the air in contact with the wood pulp when the latter is hygroscopically in equilibrium with the air.

3,510,956 Patented May 12, 1970 In the process based on resistance measurements, roller electrodes are used to provide the measured values. Two roller electrodes, rotating in opposite direction, are located adjacent to each other but electrically isolated from each other. From the main flow stream of the wood pulp a branch is tapped off and directed through the two roller electrodes. Such an arrangement, however, is particularly dependent upon the uniformity of the stream through the rollers. Such uniformity in feeding the material to the measuring instrumentation is attainable only when the material is finely pulverized or is of small particles. It is not feasible with wood shavings. When using such wood shavings in practice, the measured values vary to the extent that it is not possible to use them for regulating purposes. Another diificulty associated with this process resides in the condition that the shavings at the output of the dryer are negatively charged when the moisture content is below 10% The measurement of the dielectric constant is accomplished through the aid of plate capacitors or so-called stray field capacitors. In this process also much difliculty is encountered with regard to the requirement for uniformity of the stream of material. Furthermore, in this process the stream density within the field of the capacitor becomes significant.

The process mentioned above under item (c) is based on the measurement of the relative air humidity which is in direct contact with the stream of woodshavings within a confined space. In this arrangement, a hygroscopic equilibrium is established between the shavings and the air. In this maner, it is possible to determine the moisture content of the shavings by measuring the relative humidity of the air. Aside from the technical difiiculties inherent in the apparatus of this process, due to dust covering of the humidity sensor and wet bulb thermometer, the process is severely lacking because of its large inertia characteristic. Aside from this, measurements are not possible outside of the hygroscopic region as, for example, moisture contents exceeding approximately 30% in the shavings corresponding to relative air humidity at 20 C. When higher temperatures are involved even further limitations are encountered in the measuring region.

The automation of the drying process described below is realized through the creation of a newly designed measuring device. The measuring device in accordance with the present invention, is operable for moisture contents of the shavings between 2% and 100%. This newly designed measuring apparatus has also the least possible response lag and its measured results are independent of the feed rate of the stream of shavings. The results derived from the application of this new measuring apparatus make it possible to regulate the entire drying process.

The drying process operates, from the basic viewpoint, so that the shavings are brought to the dryer by means of a transmitting hopper. The hopper has an output conveyor or belt the speed of which may be regulated in a step-wise or stepless manner. The regulation is accomplished by means of a driving mechanism which drives the conveyor. The dryers are designed for continuous operation in which the shavings are continuously and steadily transported through the dryer by means of rollers. Within the dryers, the shavings are subjected to a blast of hot air by means of a jet system. The jets may be arranged and adjusted so that they may be located to provide stepless results with and against the transport direction of the shavings. By varying the position of the jets and thereby the required time of the shavings through the dryer, the moisture content of the shavings at the output of the dryer may be regulated between predetermined limits.

The heat required in the dryer so that the latter may perform its function, is conveyed to the dryer by means of an air circulating system which includes an air heaater. The water evaporated in the dryer is removed by the replacement of a predetermined portion of the recirculated air with a fresh supply of air. The introduction of fresh air into the circulating system and the replacement of a portion of the recirculated air is accomplished through means of a valve arrangement.

For the purpose of performing the measuring function, the procedure based on resistance measurements is selected, in accordance with the present invention. The measuring instrument, in this regard, is similar to the arrangement, for example, previously proposed for measuring the moisture content of flour. In this conventional and well-known arrangement, a sample of the flour is compressed within a tube by means of an anger. Thus, the auger receives, on a continuous basis, flour and compresses the latter so as to form a sample disk of flour within the tube containing the auger. A resistance measurement is then taken across the compressed flour disk. The opening at the exit of the space at which the sample flour is subjected to measurement, is closed in the conventional arrangement by means of a coneshaped valve in which the needle point of the valve protrudes into the measuring space. This cone-shaped valve or needle valve is maintained closed by means of the pressure exerted upon it through a spring. As a result of the pressure exerted by the auger, a small space is generated between the external surface of the needle valve and the rim of the exit opening. The flour can thereby exit through this small space. Within the measuring cylinder, a constant pressure is also generated, in this manner. This constant pressure is maintained even when no material is being conveyed through the cylinder. However, in view of the condition that measurements on wood shavings are involved rather than flour particles, this arrangement is not suitable for the purpose intended by the present invention.

In accordance with the present invention, the measuring cylinder has, at its exit end, one or more narrowly bent leaf springs. The intended purpose of the latter is to form a shavings plug at the initial filling of the instrument cylinder. Once this plug has been formed, the leaf springs become horizontal and have no further influence upon the operation of the measuring instrument. The shavings plug is generated within the measuring cylinder as a result of the rubbing action between the shavings and the cylinder wall. In this manner, a material sample is obtained which is relatively homogeneous, of uniform pressure throughout, and independent of the mass of material being tranferred by the auger.

An arrangement of this type permits the moisture content of shavings to be measured down to For moisture contents below this value of 10%, measurements are not possible without further means due to the electrostatic charging of the shavings. It is therefore necessary to eliminate this electrostatic charging in order to realize precise measurements. In accordance with the present invention, the elimination of this undesirable electrostatic condition may be accomplished in two Ways. In one process, the electrostatic charge is conducted away by means of a conducting zone generated through an electrically ionic beam. In another proces, the discharge of the shavings is accomplished through opositely polarized rubbing effect.

In the first process for eliminating the electrostatic charge from the shavings, the shavings are transferred into a space prior to the measuring instrument, wherein an ionized beam, particularly a corpuscle beam, prevails. This ionized beam causes the air to be conductive. In this beam covered space, the static charge is conducted away through the ionized air.

In the second method for eliminating the electrostatic discharge, the shavings are subjected to a rubbing action against a synthetic material or the like, before the shavings are conveyed into the measuring zone. The synthetic substance which acquires a negative charge from the wood shavings as a result of this action, is neutralized by means of a positive charge. The residual positive or negative charge remaining on the wood shavings are then conducted away by establishing contact between the shavings and a metallic cylindrical wall.

In accordance with the preferred embodiment to be described, the exterior zone of the measuring cylinder containing the material plug, is neutralized through the rubbing action against a synthetic material ring along the cylinder wall. In this manner, the electrical resistance measurement is made feasible. The measurement is performed through, for example, cup-shaped electrodes made of insulating ceramics. These electrodes project into the measuring space by a few millimeters. The measuring distance is established between one or more parallel connected electrodes and the cylinder wall. For purposes of taking the measurement therefore, only the neutralized exterior layer at the cylinder wall of the measuring plug is used.

SUMMARY OF THE INVENTION A process for drying wood shavings in the manufacture of pressed boards. A conveyor carries the wood shavings through the process chamber in which it is dried. The moisture content of the wood shavings is measured at the beginning and end of the drying process. The temperature of the process chamber is adjusted as a function of the moisture content of the wood shavings at the beginning of the process. The speed at which the wood shavings are conveyed through the process chamber is regulated as a function of the temperature at the end of the process. A mass of air is circulated through the process chamber is regulated as a function of the temperature at the end of the process. A mass of air is circulated through the process chamber and brought into contact with the wood shavings to the extent that the moisture of the air is in equilibrium with the moisture of the wood shavings. A fresh supply amount of air is added to this mass of air in a regulated manner so as to control the climate of the process as a function of the equilibrium moisture.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic representation of the instrument for measuring the humidity of the Wood shavings; and

FIG. 2 is a functional block diagram of the process for drying wood shavings, in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, and in particular to FIG. 1, the wood shavings, splinters or sawdust are contained within the storage bin or storage shaft 1a. Within this storage shaft is a deflecting blade or member 2a rotatable about a horizontal axis. The deflecting blade 2a deflects the dropping sawdust or shavings so that a steady stream is directed into the feed opening 3a of the measuring cylinder 4a. The main stream of wood shavings or sawdust passing through the feed opening 3a falls upon an anger 5a rotatable within the cylinder 4a of the measuring instrument. The auger 5a is mechanically coupled to the shaft of a motor 6a so that it may be driven by the motor. At the exit or outlet of the cylinder 4a of the measuring instrument, is situated a semicircular-shaped leaf spring 7a. The leaf spring 7a produces a damming effect by serving as a plug at the beginning of the measuring process. This plugging action of the leaf spring 7a is maintained until the shavings or sawdust completely fill the cross-section of the cylinder 4a. Measuring electrodes 8a and 9a are located diametrically opposite to each other on the cylinder. A second pair of electrodes may be similarly located on the cylinder and displaced, for example, 90 degrees, from the two electrodes 8a and 9a which are shown.

Within the measuring cylinder 40 is a ring 1012 made of high insulating material. This ring 10a is indicated in the drawing by broken lines. A signal amplifier 11a is mounted at the upper portion of the storage bin 1a: at the exterior wall thereof. At the lower portion of the bin 1a, a temperature-sensing device 12a is situated. Through these elements the required temperature compensation is applied to the measured value in an automatic manner, so that the measured value may be directly fed to an associated regulator.

This measuring instrument determines first the moisture content of the sawdust or wood shavings, with the required accuracy and certainty. The measurement is taken throughout the region of interest, and the drying process is regulated as a result of the measurements attained. The regulation of the drying process is made dependent upon the moisture content of the sawdust at the input as well as at the output of the process. An example of a regulating arrangement, in accordance with the present invention, is shown in the block diagram of FIG. 2.

From a hopper 2 of the dryer 1, the sawdust, shavings or chips, fall freely into the measuring instrument or apparatus 3, by branching off from the main flow line. At this point, the shavings or chips have their initial moisture content. The measured results of the measuring instrument 3 are suitably amplified by an amplifier 4 and transmitted to the regulator 5. This regulator possesses, for example, two adjustable contact marks by which the region of measurement may be subdivided into three moisture zones. When the measured value shifts from one moisture zone to another, signals are transmitted from the regulator to the temperature regulator 6 to increase or reduce the temperature as required. At the same time, the regulator 5 also transmits signals to the driving motor 10, by way of the stepping switch or stepping regulator 9. The driving motor is connected to the hopper and the speed of the motor determines the rate of feed of material from the hopper. Thus, a. higher or lower speed of the motor results in a greater or lesser amount of material being supplied to the system by the hopper. The speed of the motor is adjusted by the step regulator 9 which may be constructed in the form of the conventional rheostat. The position of the rheostat 9 is then determined by the signals realized from the regulator 5. Henceforth, the speed of the hopper is regulated as a function of moisture content through the regulating loop including the measuring instrument 3, amplifier 4, regulator 5, motor contro19 and driving motor 10.

The temperature measurement in the dryer is accomplished through the temperature sensor 7. The measured value from the temperature sensor 7 is transmitted to the temperature regulator and indicator 6, where the measured temperature is indicated. The temperature is then regulated through the action of a motorized valve 8. The motorized valve receives its positioning signals from the temperature regulator and indicator 6. The latter possesses three adjustable marks arranged to correspond to each one of the moisture zones of the regulator 5. It is quite possible, through the arrangement of the loop including the temperature sensor 7, regulator 6 and valve 8, to regulate the temperature in a proportional manner with respect to the moisture content of the sawdust, shavings or chips at the input.

The moisture content of the chips or shavings at the output of the process is determined by the moisture measuring instrument 11. The resulting measured value is transmitted to an amplifier 12 which also has provision for an input from a temperature sensor 13. The amplifier 12 utilizes the signal from the temperature sensor 13 to compensate the measured value from the moisture measuring instrument 11 with respect to temperature. The compensated content value is then transmitted by the amplifier 12 to the regulator 14. The regulator 14 possesses two markings whereby it is possible to set the tolerable limits of the moisture content at the output of the drive. When the compensated value from the amplifier 12 is above or below the limits set at the regulator 14, setting signals are transmitted to the setting apparatus 15 which sets the dryer nozzle or dryer jet feed. The arrangement is such that when the compensated and measured value from the amplifier 12 exceeds the upper limit, the setting apparatus 15 adjusts the dryer feed so as to decrease the flow therethrough. When the measured and compensated value from the amplifier 12 is, on the other hand, below the lower limit, the setting apparatus 15 operates so as to increase the flow through the dryer jet. The regulator 14 transmits simultaneously signals to the motor control 9 for adjusting the operating speed of the motor 10 and thus adjust the hopper feed in accordance with the proper requirements.

The regulation of the equilibrium state of the moisture in the outgoing air from the dryer, is accomplished by measuring the equilibrium moisture of the wood by mean-s of the instrument 60. This moisture equilibrium measuring instrument 16 is located at the recirculation line. The measured value from the output of the instrument 16 is transmitted to the amplifier 17 Where it is compensated against temperature by means of the value obtained from the temperature sensor 18. The resulting compensated value from the amplifier 17, is then passed on to the regulator 19. The latter possesses a settable mark which may be set to the desired characteristic of the recirculated air. For the purpose of affecting the outgoing air as well as the freshly supplied air, as a function of the signal output of amplifier 17, the regulator 19 examines the output from amplifier 17 to determine whether it is above or below the set mark. As a consequence of this action of the regulator 19, signals are transmitted to the motor controlled flap-type of valve 20. In this manner the characteristics of the outgoing air as well as the freshly supplied air are established.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of automatic regulating arrangements for the moisture content of sawdust, differing from the types described above.

While the invention has been illustrated and described as embodied in automatic regulating arrangements for the moisture content of sawdust, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can be applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. A process for drying wood shavings comprising the steps of conveying said wood shavings to be processed to a processing chamber; transporting said wood shavings through said processing chamber; measuring the moisture content of said wood shavings at the beginning of said process; adjusting the temperature of said process as a function of the moisture content of said shavings at the beginning of said process so that the moisture content decreases in a predetermined manner while said wood shavings are transported through said chamber; measuring the moisture content of the wood shavings at the end of said process; regulating the speed at which said wood shavings are conveyed through said process as a function of the moisture content of said wood shavings at the end of said process so that said wood shavings have a moisture content below a predetermined level at the end of said process; passing a mass of air through said wood shavings and in contact therewith so that the moisture in the air is in equilibrium with the moisture in the shavings; and adding a fresh supply of air to said mass of air so as to regulate the humidity of said mass of air as a function of the moisture in the mass of air when in equilibruim with the moisture content of said wood shavings, the moisture in the mixture of said mass of air and said fresh supply of air affecting the moisture content of said wood shavings so that said wood shavings retain a predetermined amount of moisture.

2. The process for drying wood shavings as defined in claim 1 including the step of measuring the temperature at the beginning and end of said process.

3. The process for drying wood shavings as defined in claim 2 including the step of regulating the speed at which said wood shavings are conveyed through said process as a function of the temperature at the beginning of said process.

4. The process for drying wood shavings as defined in claim 1, including the step of regulating the speed at which said wood shavings are conveyed to said processing chamber as a function of the moisture content of said woodshavings at the beginning of said process.

5. The process for drying wood shavings as defined in claim 1, including the step of regulating the speed at which said wood shavings are conveyed to said processing chamber as a function of the moisture content of said wood shavings at the beginning of said process.

6. The process for drying wood shavings as defined in claim 1, including the step of regulating the speed at which said wood shavings are conveyed to said processing chamber as a function of the moisture content of said wood shavings at the end of said process.

7. A process for drying wood shavings comprising the steps of conveying said wood shavings to be processed to a processing chamber; transporting said wood shavings through said processing chamber; measuring the moisture content of said wood shavings at the beginning of said process; adjusting the speed at which said wood shavings are conveyed to said processing chamber as a function of the moisture content of said shavings at the beginning of said process so that said wood shavings have a moisture content below a predetermined level at the end of said process; measuring the moisture content of the wood shavings at the end of said process; regulating the speed at which said wood shavings are conveyed to said processing chamber as a function of the moisture content of said wood shavings at the end of said process so that the moisture content decreases in a predetermined manner while said wood shavings are transported through said chamber; passing a mass of air through said wood shavings and in contact therewith so that the moisture in the air is in equilibrium with the moisture in the shavings; and adding a fresh supply of air to said mass of air so as to regulate the humidity of said mass of air as a function of the moisture in the mass of air when in equilibrium with the moisture content of said wood shavings, the moisture in the mixture of said mass of air and said fresh supply of air affecting the moisture content of said wood shavings so that said wood shavings retain a predetermined amount of moisture.

References Cited UNITED STATES PATENTS 2,078,515 4/ 1937 Sutherland 3425 2,184,473 12/1939 Scanlan 3446 2,768,629 10/1956 Maul 34-46 X 3,372,488 3/1968 Koch et a1 3448 X 3,396,476 6/1968 Eaves 3452 X 3,259,995 7/ 1966 Powischill 3431 3,367,038 2/1968 Bishop 3431 JOHN J. CAMBY, Primary Examiner 

